Various types of structures include generally cylindrical tubular members. It is often a requirement of such structures that at least a portion of the outside surface of a tube member present a generally smooth cylindrical surface. For example, in the area of structures used in sporting contests, the horizontal and upright members of an American football goal should preferably present smooth surfaces. The same is true for a soccer goal. Smooth cylindrical surfaces are required to either deflect a ball away from the goal or into the goal in a predictable manner. Another consideration for such structures is that load bearing members should remain substantially straight and have no significant deflection. This requirement may be present for a wide range of structures, even structures that do not require smooth, cylindrical surfaces. This requirement for limited deflection leads designers to use long, straight, strong and uninterrupted members that are free of joints. Yet, a structure is much easier to transport and use if it is possible to disassemble the structure. This is particularly the case for structures having long straight members. Yet another consideration for structures of various types and particularly for soccer goals, is that it is desirable to combine members of different lengths to make structures of different sizes. When tubular members are joined to make longer segments, segments of different lengths may be used to make structures of different sizes. This would be particularly advantageous in the case of soccer goals which are constructed to conform to a series of regulation sizes for various age groups. Accordingly, what is needed is a system for joining tubular members which can be used to join relatively short lengths of structural tubing in a manner which provides strong and rigid joints and which. In the case of structures for sporting contests, a joining system is also needed to provide joints which present smooth surfaces for those portions of a joint that are in play in a contest. And also, particularly in the case of structures for sporting contest, a joining system is needed for making structures of various sizes.